1. Field of the Invention
The present invention relates to a process for producing a modified hydroxyethyl cellulose having excellent heat resistant properties and substantially no pungent smell (or irritating odor) when dissolved.
2. Description of the Related Art
Hydroxyethyl cellulose has been widely used in various industrial fields as, for example, a stabilizer for emulsion polymerization, as a dispersant for pigment in, for example, coating compositions, as a binder, a sizing agent, and a surface coating agent in paper industries, as a thickening agent in cosmetics, and as a water retention agent for cement. Furthermore, hydroxyethyl cellulose has attracted attention, in oil and gas explorations, as a viscosifier for drilling mud, well stimulating fluid, cementing fluid, completion fluid, workover fluid, or as a mobility control agent for polymer flooding or as a mobility buffer for micellar/polymer flooding in enhanced oil recovery (EOR). Recently, the exploration of oil are deeper, and thus the temperature of the oil layers is generally 40.degree. C. to 80.degree. C.; sometimes up to 150.degree. C. at high temperature oil wells. Accordingly, the viscosifier having excellent heat resistant properties, which does not cause an unpreferable decrease in the viscosity even at an elevated temperature, are urgently required.
Known in the art is a method for obtaining a viscosifier having excellent heat resistant properties wherein various stabilizers are added to polyacrylamides. For example, it is disclosed in Japanese Examined Patent Publication (Kokoku) Nos. 58-47414 and 58-48583 that 2-mercapto benzimidazole compounds are used as a stabilizer. Furthermore, as a method for improving the stability of hydroxyethyl cellulose, it has been proposed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 58-113201, that methyl chloride or monochloroacetic acid is added to a reaction mixture of cellulose and ethylene oxide. This method, however, can improve the stability only at room temperature.
The present inventors previously found that the decrease in the rheological characteristics, at an elevated temperature, of a conventional chemical fluid composition for well improvement comprising at least one salt selected from the group consisting of calcium chloride, calcium bromide, and zinc bromide, hydroxyethyl cellulose (HEC), and water can be suppressed by including a 2-mercapto benzimidazole compound (MBI), as proposed in Japanese Patent Application No. 59-63402 (i.e., USSN 719,391 filed on April 2, 1985, now U.S. Pat. No. 4,614,601). However, not only MBI itself but also HEC/MBI previously mixed in the form of powder disadvantageously have a pungent smell, which is not preferable from an operational viewpoint. Although this problem can be solved by automatically (or nonmanually) carrying out the operation in a closed system, the automatic operation is technically and economically difficult because the operation must be locally carried out at each oil well. Thus, a manual operation must be carried out and, therefore, the removal of the pungent smell is strongly desired.